Use of data latches in cache operations of non-volatile memories

ABSTRACT

Methods and circuitry are present for improving performance in non-volatile memory devices by allowing the inter-phase pipelining of operations with the same memory, allowing, for example, a read operation to be interleaved between the pulse and verify phases of a write operation. In the exemplary embodiment, the two operations share data latches. In specific examples, at the data latches needed for verification in a multi-level write operation free up, they can be used to store data read from another location during a read performed between steps in the multi-level write. In the exemplary embodiment, the multi-level write need only pause, execute the read, and resume the write at the point where it paused.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 11/619,513,filed on Jan. 3, 2007, now U.S. Pat. No. 7,577,037, which is acontinuation of application Ser. No. 11/097,590, filed on Apr. 1, 2005,now U.S. Pat. No. 7,206,230, which applications are incorporated hereinin their entirety by this reference.

This application is related to the following U.S. patent applications:application Ser. No. 11/013,125, filed Dec. 14, 2004, now U.S. Pat. No.7,120,051; application Ser. No. 11/026,536, filed Dec. 29, 2004, nowpublished as US PG Pub 2006/0140007; application Ser. No. 11/015,199,filed Dec. 16, 2004, now U.S. Pat. No. 7,046,568; filed Mar. 16, 2005;and application Ser. No. 11/097,517, filed Apr. 1, 2005. Theseapplications are incorporated herein by reference in their entirety forall purposes.

FIELD OF THE INVENTION

This invention relates generally to non-volatile semiconductor memorysuch as electrically erasable programmable read-only memory (EEPROM) andflash EEPROM, and specifically to cache operations based on shared latchstructures allowing overlapping memory operations.

BACKGROUND OF THE INVENTION

Solid-state memory capable of nonvolatile storage of charge,particularly in the form of EEPROM and flash EEPROM packaged as a smallform factor card, has recently become the storage of choice in a varietyof mobile and handheld devices, notably information appliances andconsumer electronics products. Unlike RAM (random access memory) that isalso solid-state memory, flash memory is non-volatile, retaining itsstored data even after power is turned off. In spite of the higher cost,flash memory is increasingly being used in mass storage applications.Conventional mass storage, based on rotating magnetic medium such ashard drives and floppy disks, is unsuitable for the mobile and handheldenvironment. This is because disk drives tend to be bulky, are prone tomechanical failure and have high latency and high power requirements.These undesirable attributes make disk-based storage impractical in mostmobile and portable applications. On the other hand, flash memory, bothembedded and in the form of a removable card is ideally suited in themobile and handheld environment because of its small size, low powerconsumption, high speed and high reliability features.

EEPROM and electrically programmable read-only memory (EPROM) arenon-volatile memory that can be erased and have new data written or“programmed” into their memory cells. Both utilize a floating(unconnected) conductive gate, in a field effect transistor structure,positioned over a channel region in a semiconductor substrate, betweensource and drain regions. A control gate is then provided over thefloating gate. The threshold voltage characteristic of the transistor iscontrolled by the amount of charge that is retained on the floatinggate. That is, for a given level of charge on the floating gate, thereis a corresponding voltage (threshold) that must be applied to thecontrol gate before the transistor is turned “on” to permit conductionbetween its source and drain regions.

The floating gate can hold a range of charges and therefore can beprogrammed to any threshold voltage level within a threshold voltagewindow. The size of the threshold voltage window is delimited by theminimum and maximum threshold levels of the device, which in turncorrespond to the range of the charges that can be programmed onto thefloating gate. The threshold window generally depends on the memorydevice's characteristics, operating conditions and history. Eachdistinct, resolvable threshold voltage level range within the windowmay, in principle, be used to designate a definite memory state of thecell.

The transistor serving as a memory cell is typically programmed to a“programmed” state by one of two mechanisms. In “hot electroninjection,” a high voltage applied to the drain accelerates electronsacross the substrate channel region. At the same time a high voltageapplied to the control gate pulls the hot electrons through a thin gatedielectric onto the floating gate. In “tunneling injection,” a highvoltage is applied to the control gate relative to the substrate. Inthis way, electrons are pulled from the substrate to the interveningfloating gate.

The memory device may be erased by a number of mechanisms. For EPROM,the memory is bulk erasable by removing the charge from the floatinggate by ultraviolet radiation. For EEPROM, a memory cell is electricallyerasable, by applying a high voltage to the substrate relative to thecontrol gate so as to induce electrons in the floating gate to tunnelthrough a thin oxide to the substrate channel region (i.e.,Fowler-Nordheim tunneling.) Typically, the EEPROM is erasable byte bybyte. For flash EEPROM, the memory is electrically erasable either allat once or one or more blocks at a time, where a block may consist of512 bytes or more of memory.

Examples of Non-Volatile Memory Cells

The memory devices typically comprise one or more memory chips that maybe mounted on a card. Each memory chip comprises an array of memorycells supported by peripheral circuits such as decoders and erase, writeand read circuits. The more sophisticated memory devices also come witha controller that performs intelligent and higher level memoryoperations and interfacing. There are many commercially successfulnon-volatile solid-state memory devices being used today. These memorydevices may employ different types of memory cells, each type having oneor more charge storage element.

FIGS. 1A-1E illustrate schematically different examples of non-volatilememory cells.

FIG. 1A illustrates schematically a non-volatile memory in the form ofan EEPROM cell with a floating gate for storing charge. An electricallyerasable and programmable read-only memory (EEPROM) has a similarstructure to EPROM, but additionally provides a mechanism for loadingand removing charge electrically from its floating gate upon applicationof proper voltages without the need for exposure to UV radiation.Examples of such cells and methods of manufacturing them are given inU.S. Pat. No. 5,595,924.

FIG. 1B illustrates schematically a flash EEPROM cell having both aselect gate and a control or steering gate. The memory cell 10 has a“split-channel” 12 between source 14 and drain 16 diffusions. A cell isformed effectively with two transistors T1 and T2 in series. T1 servesas a memory transistor having a floating gate 20 and a control gate 30.The floating gate is capable of storing a selectable amount of charge.The amount of current that can flow through the T1's portion of thechannel depends on the voltage on the control gate 30 and the amount ofcharge residing on the intervening floating gate 20. T2 serves as aselect transistor having a select gate 40. When T2 is turned on by avoltage at the select gate 40, it allows the current in the T1's portionof the channel to pass between the source and drain. The selecttransistor provides a switch along the source-drain channel independentof the voltage at the control gate. One advantage is that it can be usedto turn off those cells that are still conducting at zero control gatevoltage due to their charge depletion (positive) at their floatinggates. The other advantage is that it allows source side injectionprogramming to be more easily implemented.

One simple embodiment of the split-channel memory cell is where theselect gate and the control gate are connected to the same word line asindicated schematically by a dotted line shown in FIG. 1B. This isaccomplished by having a charge storage element (floating gate)positioned over one portion of the channel and a control gate structure(which is part of a word line) positioned over the other channel portionas well as over the charge storage element. This effectively forms acell with two transistors in series, one (the memory transistor) with acombination of the amount of charge on the charge storage element andthe voltage on the word line controlling the amount of current that canflow through its portion of the channel, and the other (the selecttransistor) having the word line alone serving as its gate. Examples ofsuch cells, their uses in memory systems and methods of manufacturingthem are given in U.S. Pat. Nos. 5,070,032, 5,095,344, 5,315,541,5,343,063, and 5,661,053.

A more refined embodiment of the split-channel cell shown in FIG. 1B iswhen the select gate and the control gate are independent and notconnected by the dotted line between them. One implementation has thecontrol gates of one column in an array of cells connected to a control(or steering) line perpendicular to the word line. The effect is torelieve the word line from having to perform two functions at the sametime when reading or programming a selected cell. Those two functionsare (1) to serve as a gate of a select transistor, thus requiring aproper voltage to turn the select transistor on and off, and (2) todrive the voltage of the charge storage element to a desired levelthrough an electric field (capacitive) coupling between the word lineand the charge storage element. It is often difficult to perform both ofthese functions in an optimum manner with a single voltage. With theseparate control of the control gate and the select gate, the word lineneed only perform function (1), while the added control line performsfunction (2). This capability allows for design of higher performanceprogramming where the programming voltage is geared to the targeteddata. The use of independent control (or steering) gates in a flashEEPROM array is described, for example, in U.S. Pat. Nos. 5,313,421 and6,222,762.

FIG. 1C illustrates schematically another flash EEPROM cell having dualfloating gates and independent select and control gates. The memory cell10 is similar to that of FIG. 1B except it effectively has threetransistors in series. In this type of cell, two storage elements (i.e.,that of T1-left and T1-right) are included over its channel betweensource and drain diffusions with a select transistor T1 in between them.The memory transistors have floating gates 20 and 20′, and control gates30 and 30′, respectively. The select transistor T2 is controlled by aselect gate 40. At any one time, only one of the pair of memorytransistors is accessed for read or write. When the storage unit T1-leftis being accessed, both the T2 and T1-right are turned on to allow thecurrent in the T1-left's portion of the channel to pass between thesource and the drain. Similarly, when the storage unit T1-right is beingaccessed, T2 and T1-left are turned on. Erase is effected by having aportion of the select gate polysilicon in close proximity to thefloating gate and applying a substantial positive voltage (e.g. 20V) tothe select gate so that the electrons stored within the floating gatecan tunnel to the select gate polysilicon.

FIG. 1D illustrates schematically a string of memory cells organizedinto an NAND cell. An NAND cell 50 consists of a series of memorytransistors M1, M2, . . . Mn (n=4, 8, 16 or higher) daisy-chained bytheir sources and drains. A pair of select transistors S1, S2 controlsthe memory transistors chain's connection to the external via the NANDcell's source terminal 54 and drain terminal 56. In a memory array, whenthe source select transistor S1 is turned on, the source terminal iscoupled to a source line. Similarly, when the drain select transistor S2is turned on, the drain terminal of the NAND cell is coupled to a bitline of the memory array. Each memory transistor in the chain has acharge storage element to store a given amount of charge so as torepresent an intended memory state. A control gate of each memorytransistor provides control over read and write operations. A controlgate of each of the select transistors S1, S2 provides control access tothe NAND cell via its source terminal 54 and drain terminal 56respectively.

When an addressed memory transistor within an NAND cell is read andverified during programming, its control gate is supplied with anappropriate voltage. At the same time, the rest of the non-addressedmemory transistors in the NAND cell 50 are fully turned on byapplication of sufficient voltage on their control gates. In this way, aconductive path is effective created from the source of the individualmemory transistor to the source terminal 54 of the NAND cell andlikewise for the drain of the individual memory transistor to the drainterminal 56 of the cell. Memory devices with such NAND cell structuresare described in U.S. Pat. Nos. 5,570,315, 5,903,495, 6,046,935.

FIG. 1E illustrates schematically a non-volatile memory with adielectric layer for storing charge. Instead of the conductive floatinggate elements described earlier, a dielectric layer is used. Such memorydevices utilizing dielectric storage element have been described byEitan et al., “NROM: A Novel Localized Trapping, 2-Bit NonvolatileMemory Cell,” IEEE Electron Device Letters, vol. 21, no. 11, Nov. 2000,pp. 543-545. An ONO dielectric layer extends across the channel betweensource and drain diffusions. The charge for one data bit is localized inthe dielectric layer adjacent to the drain, and the charge for the otherdata bit is localized in the dielectric layer adjacent to the source.For example, U.S. Pat. Nos. 5,768,192 and 6,011,725 disclose anonvolatile memory cell having a trapping dielectric sandwiched betweentwo silicon dioxide layers. Multi-state data storage is implemented byseparately reading the binary states of the spatially separated chargestorage regions within the dielectric.

Memory Array

A memory device typically comprises of a two-dimensional array of memorycells arranged in rows and columns and addressable by word lines and bitlines. The array can be formed according to an NOR type or an NAND typearchitecture.

NOR Array

FIG. 2 illustrates an example of an NOR array of memory cells. Memorydevices with an NOR type architecture have been implemented with cellsof the type illustrated in FIG. 1B or 1C. Each row of memory cells areconnected by their sources and drains in a daisy-chain manner. Thisdesign is sometimes referred to as a virtual ground design. Each memorycell 10 has a source 14, a drain 16, a control gate 30 and a select gate40. The cells in a row have their select gates connected to word line42. The cells in a column have their sources and drains respectivelyconnected to selected bit lines 34 and 36. In some embodiments where thememory cells have their control gate and select gate controlledindependently, a steering line 36 also connects the control gates of thecells in a column.

Many flash EEPROM devices are implemented with memory cells where eachis formed with its control gate and select gate connected together. Inthis case, there is no need for steering lines and a word line simplyconnects all the control gates and select gates of cells along each row.Examples of these designs are disclosed in U.S. Pat. Nos. 5,172,338 and5,418,752. In these designs, the word line essentially performed twofunctions: row selection and supplying control gate voltage to all cellsin the row for reading or programming.

NAND Array

FIG. 3 illustrates an example of an NAND array of memory cells, such asthat shown in FIG. 1D. Along each column of NAND cells, a bit line iscoupled to the drain terminal 56 of each NAND cell. Along each row ofNAND cells, a source line may connect all their source terminals 54.Also the control gates of the NAND cells along a row are connected to aseries of corresponding word lines. An entire row of NAND cells can beaddressed by turning on the pair of select transistors (see FIG. 1D)with appropriate voltages on their control gates via the connected wordlines. When a memory transistor within the chain of a NAND cell is beingread, the remaining memory transistors in the chain are turned on hardvia their associated word lines so that the current flowing through thechain is essentially dependent upon the level of charge stored in thecell being read. An example of an NAND architecture array and itsoperation as part of a memory system is found in U.S. Pat. Nos.5,570,315, 5,774,397 and 6,046,935.

Block Erase

Programming of charge storage memory devices can only result in addingmore charge to its charge storage elements. Therefore, prior to aprogram operation, existing charge in a charge storage element must beremoved (or erased). Erase circuits (not shown) are provided to eraseone or more blocks of memory cells. A non-volatile memory such as EEPROMis referred to as a “Flash” EEPROM when an entire array of cells, orsignificant groups of cells of the array, is electrically erasedtogether (i.e., in a flash). Once erased, the group of cells can then bereprogrammed. The group of cells erasable together may consist one ormore addressable erase unit. The erase unit or block typically storesone or more pages of data, the page being the unit of programming andreading, although more than one page may be programmed or read in asingle operation. Each page typically stores one or more sectors ofdata, the size of the sector being defined by the host system. Anexample is a sector of 512 bytes of user data, following a standardestablished with magnetic disk drives, plus some number of bytes ofoverhead information about the user data and/or the block in with it isstored.

Read/Write Circuits

In the usual two-state EEPROM cell, at least one current breakpointlevel is established so as to partition the conduction window into tworegions. When a cell is read by applying predetermined, fixed voltages,its source/drain current is resolved into a memory state by comparingwith the breakpoint level (or reference current I_(REF)). If the currentread is higher than that of the breakpoint level, the cell is determinedto be in one logical state (e.g., a “zero” state). On the other hand, ifthe current is less than that of the breakpoint level, the cell isdetermined to be in the other logical state (e.g., a “one” state). Thus,such a two-state cell stores one bit of digital information. A referencecurrent source, which may be externally programmable, is often providedas part of a memory system to generate the breakpoint level current.

In order to increase memory capacity, flash EEPROM devices are beingfabricated with higher and higher density as the state of thesemiconductor technology advances. Another method for increasing storagecapacity is to have each memory cell store more than two states.

For a multi-state or multi-level EEPROM memory cell, the conductionwindow is partitioned into more than two regions by more than onebreakpoint such that each cell is capable of storing more than one bitof data. The information that a given EEPROM array can store is thusincreased with the number of states that each cell can store. EEPROM orflash EEPROM with multi-state or multi-level memory cells have beendescribed in U.S. Pat. No. 5,172,338.

In practice, the memory state of a cell is usually read by sensing theconduction current across the source and drain electrodes of the cellwhen a reference voltage is applied to the control gate. Thus, for eachgiven charge on the floating gate of a cell, a corresponding conductioncurrent with respect to a fixed reference control gate voltage may bedetected. Similarly, the range of charge programmable onto the floatinggate defines a corresponding threshold voltage window or a correspondingconduction current window.

Alternatively, instead of detecting the conduction current among apartitioned current window, it is possible to set the threshold voltagefor a given memory state under test at the control gate and detect ifthe conduction current is lower or higher than a threshold current. Inone implementation the detection of the conduction current relative to athreshold current is accomplished by examining the rate the conductioncurrent is discharging through the capacitance of the bit line.

FIG. 4 illustrates the relation between the source-drain current I_(D)and the control gate voltage V_(CG) for four different charges Q1-Q4that the floating gate may be selectively storing at any one time. Thefour solid I_(D) versus V_(CG) curves represent four possible chargelevels that can be programmed on a floating gate of a memory cell,respectively corresponding to four possible memory states. As anexample, the threshold voltage window of a population of cells may rangefrom 0.5V to 3.5V. Six memory states may be demarcated by partitioningthe threshold window into five regions in interval of 0.5V each. Forexample, if a reference current, I_(REF) of 2 μA is used as shown, thenthe cell programmed with Q1 may be considered to be in a memory state“1” since its curve intersects with I_(REF) in the region of thethreshold window demarcated by V_(CG)=0.5V and 1.0V. Similarly, Q4 is ina memory state “5”.

As can be seen from the description above, the more states a memory cellis made to store, the more finely divided is its threshold window. Thiswill require higher precision in programming and reading operations inorder to be able to achieve the required resolution.

U.S. Pat. No. 4,357,685 discloses a method of programming a 2-stateEPROM in which when a cell is programmed to a given state, it is subjectto successive programming voltage pulses, each time adding incrementalcharge to the floating gate. In between pulses, the cell is read back orverified to determine its source-drain current relative to thebreakpoint level. Programming stops when the current state has beenverified to reach the desired state. The programming pulse train usedmay have increasing period or amplitude.

Prior art programming circuits simply apply programming pulses to stepthrough the threshold window from the erased or ground state until thetarget state is reached. Practically, to allow for adequate resolution,each partitioned or demarcated region would require at least about fiveprogramming steps to transverse. The performance is acceptable for2-state memory cells. However, for multi-state cells, the number ofsteps required increases with the number of partitions and therefore,the programming precision or resolution must be increased. For example,a 16-state cell may require on average at least 40 programming pulses toprogram to a target state.

FIG. 5 illustrates schematically a memory device with a typicalarrangement of a memory array 100 accessible by read/write circuits 170via row decoder 130 and column decoder 160. As described in connectionwith FIGS. 2 and 3, a memory transistor of a memory cell in the memoryarray 100 is addressable via a set of selected word line(s) and bitline(s). The row decoder 130 selects one or more word lines and thecolumn decoder 160 selects one or more bit lines in order to applyappropriate voltages to the respective gates of the addressed memorytransistor. Read/write circuits 170 are provided to read or write(program) the memory states of addressed memory transistors. Theread/write circuits 170 comprise a number of read/write modulesconnectable via bit lines to memory elements in the array.

FIG. 6A is a schematic block diagram of an individual read/write module190. Essentially, during read or verify, a sense amplifier determinesthe current flowing through the drain of an addressed memory transistorconnected via a selected bit line. The current depends on the chargestored in the memory transistor and its control gate voltage. Forexample, in a multi-state EEPROM cell, its floating gate can be chargedto one of several different levels. For a 4-level cell, it may be usedto store two bits of data. The level detected by the sense amplifier isconverted by a level-to-bits conversion logic to a set of data bits tobe stored in a data latch.

Factors Affecting Read/Write Performance and Accuracy

In order to improve read and program performance, multiple chargestorage elements or memory transistors in an array are read orprogrammed in parallel. Thus, a logical “page” of memory elements areread or programmed together. In existing memory architectures, a rowtypically contains several interleaved pages. All memory elements of apage will be read or programmed together. The column decoder willselectively connect each one of the interleaved pages to a correspondingnumber of read/write modules. For example, in one implementation, thememory array is designed to have a page size of 532 bytes (512 bytesplus 20 bytes of overheads.) If each column contains a drain bit lineand there are two interleaved pages per row, this amounts to 8512columns with each page being associated with 4256 columns. There will be4256 sense modules connectable to read or write in parallel either allthe even bit lines or the odd bit lines. In this way, a page of 4256bits (i.e., 532 bytes) of data in parallel are read from or programmedinto the page of memory elements. The read/write modules forming theread/write circuits 170 can be arranged into various architectures.

Referring to FIG. 5, the read/write circuits 170 is organized into banksof read/write stacks 180. Each read/write stack 180 is a stack ofread/write modules 190. In a memory array, the column spacing isdetermined by the size of the one or two transistors that occupy it.However, as can be seen from FIG. 6A, the circuitry of a read/writemodule will likely be implemented with many more transistors and circuitelements and therefore will occupy a space over many columns. In orderto service more than one column among the occupied columns, multiplemodules are stacked up on top of each other.

FIG. 6B shows the read/write stack of FIG. 5 implemented conventionallyby a stack of read/write modules 190. For example, a read/write modulemay extend over sixteen columns, then a read/write stack 180 with astack of eight read/write modules can be used to service eight columnsin parallel. The read/write stack can be coupled via a column decoder toeither the eight odd (1, 3, 5, 7, 9, 11, 13, 15) columns or the eighteven (2, 4, 6, 8, 10, 12, 14, 16) columns among the bank.

As mentioned before, conventional memory devices improve read/writeoperations by operating in a massively parallel manner on all even orall odd bit lines at a time. This architecture of a row consisting oftwo interleaved pages will help to alleviate the problem of fitting theblock of read/write circuits. It is also dictated by consideration ofcontrolling bit-line to bit-line capacitive coupling. A block decoder isused to multiplex the set of read/write modules to either the even pageor the odd page. In this way, whenever one set of bit lines are beingread or programmed, the interleaving set can be grounded to minimizeimmediate neighbor coupling.

However, the interleaving page architecture is disadvantageous in atleast three respects. First, it requires additional multiplexingcircuitry. Secondly, it is slow in performance. To finish read orprogram of memory cells connected by a word line or in a row, two reador two program operations are required. Thirdly, it is also not optimumin addressing other disturb effects such as field coupling betweenneighboring charge storage elements at the floating gate level when thetwo neighbors are programmed at different times, such as separately inodd and even pages.

The problem of neighboring field coupling becomes more pronounced withever closer spacing between memory transistors. In a memory transistor,a charge storage element is sandwiched between a channel region and acontrol gate. The current that flows in the channel region is a functionof the resultant electric field contributed by the field at the controlgate and the charge storage element. With ever increasing density,memory transistors are formed closer and closer together. The field fromneighboring charge elements then becomes significant contributor to theresultant field of an affected cell. The neighboring field depends onthe charge programmed into the charge storage elements of the neighbors.This perturbing field is dynamic in nature as it changes with theprogrammed states of the neighbors. Thus, an affected cell may readdifferently at different time depending on the changing states of theneighbors.

The conventional architecture of interleaving page exacerbates the errorcaused by neighboring floating gate coupling. Since the even page andthe odd page are programmed and read independently of each other, a pagemay be programmed under one set of condition but read back under anentirely different set of condition, depending on what has happened tothe intervening page in the meantime. The read errors will become moresevere with increasing density, requiring a more accurate read operationand coarser partitioning of the threshold window for multi-stateimplementation. Performance will suffer and the potential capacity in amulti-state implementation is limited.

United States Patent Publication No. US-2004-0060031-A1 discloses a highperformance yet compact non-volatile memory device having a large blockof read/write circuits to read and write a corresponding block of memorycells in parallel. In particular, the memory device has an architecturethat reduces redundancy in the block of read/write circuits to aminimum. Significant saving in space as well as power is accomplished byredistributing the block of read/write modules into a block read/writemodule core portions that operate in parallel while interacting with asubstantially smaller sets of common portions in a time-multiplexingmanner. In particular, data processing among read/write circuits betweena plurality of sense amplifiers and data latches is performed by ashared processor.

Therefore there is a general need for high performance and high capacitynon-volatile memory. In particular, there is a need for a compactnon-volatile memory with enhanced read and program performance having animproved processor that is compact and efficient, yet highly versatilefor processing data among the read/writing circuits.

SUMMARY OF INVENTION

According to one aspect of the invention, cache operations are presentedthat allow data to be transferred in or out of a memory while theinternal memory is engaged in another operation, such as a read, programor erase. In particular, arrangements of data latches and methods oftheir use are described which allow such cache operations.

Architectures are described where data latches are shared by a number ofphysical pages. For example, read/write stacks are associated with thebit lines of the memory, which shared by multiple word lines. While oneoperation is going on in the memory, if any of these latch are free,they can cache data for future operations in the same or another wordline, saving transfer time as this can be hidden behind anotheroperation. This can improve performance by increasing the amount ofpipelining of different operations or phases of operations. In oneexample, in a cache program operation, while programming one page ofdata another page of data can be loaded in, saving on transfer time. Foranother example, in one exemplary embodiment, a read operation on oneword line is inserted into a write operation on another word line,allowing the data from the read to be transferred out of the memorywhile the data write continues on.

According to the various aspects, data from another page in the sameblock, but on a different word line, can be toggled out (to, forexample, do an ECC operation) while a write or other operation is goingon for the first page of data. This inter-phase pipelining of operationsallows the time needed for the data transfer to be hidden behind theoperation on the first page of data. More generally, this allows aportion of one operation to be inserted between phases of another,typically longer, operation. Another example would be to insert asensing operation between phases of, say, an erase operation, such asbefore an erase pulse or before a soft programming phase used as thelater part of the erase.

If a relatively long operation with different phases is being performed,a primary aspect will interpose in a quicker operation using the sharedlatches of the read/write stacks if latches available. For example, aread can be inserted into a program or erase operation, or a binaryprogram can be inserted into an erase. The primary exemplary embodimentswill toggle data in and/or out for one page during a program operationfor another page that shares the same read write stacks, where, forexample, a read of the data to be toggled out and modified is insertedinto the verify phase of the data write.

The availability of open data latches can arise in a number of ways.Generally, for a memory storing n bits per cell, n such data latcheswill be needed for each bit line; however, not all of these latches areneeded at all times. For example, in a two-bit per cell memory storingdata in an upper page/lower page format, one data latch will be neededwhile programming the lower page (with another latch used if quick passwrite is implemented). Two data latches will be needed while programmingthe upper page (with a third latch used if quick pass write isimplemented)). More generally, for memories storing multiple pages, allof the latches will be needed only when programming the highest page.This leaves the other latches available for cache operations. Further,even while writing the highest page, as the various states are removedfrom the verify phase of the write operation, latches will free up.Specifically, once only the highest state remains to be verified, only asingle latch is needed for verification purposes and the others may beused for cache operations.

An exemplary embodiment is based on a four state memory storing two-bitsper cell and having two latches for data on each bit line and oneadditional latch for quick pass write, The operations of writing thelower page, or erasing, or doing a post erase soft program are basicallya binary operation and have one of the data latches free, which can useit to cache data. Similarly, where doing an upper page or full sequencewrite, once all but the highest level has verified, only a single stateneeds to verify and the memory can free up a latch that can be used tocache data. An example of how this can be used is that when programmingone page, such as in a copy operation, a read of another page thatshares the same set of data latches, such as another word line on thesame set of bit lines, can be slipped in between program pulse andverifies of the write. The address can then be switched to the pagebeing written, allowing the write process to pick up where it left offwithout having to restart. While the write continues, the data cachedduring the interpolated read can be toggled out, checked or modified andtransferred back to be present for writing back in once the earlierwrite operation completes. This sort cache operation allows the togglingout and modification of the second page of data to be hidden behind theprogramming of the first page.

Additional features and advantages of the present invention will beunderstood from the following description of its preferred embodiments,which description should be taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1E illustrate schematically different examples of non-volatilememory cells.

FIG. 2 illustrates an example of an NOR array of memory cells.

FIG. 3 illustrates an example of an NAND array of memory cells, such asthat shown in FIG. 1D.

FIG. 4 illustrates the relation between the source-drain current and thecontrol gate voltage for four different charges Q1-Q4 that the floatinggate may be storing at any one time.

FIG. 5 illustrates schematically a typical arrangement of a memory arrayaccessible by read/write circuits via row and column decoders.

FIG. 6A is a schematic block diagram of an individual read/write module.

FIG. 6B shows the read/write stack of FIG. 5 implemented conventionallyby a stack of read/write modules.

FIG. 7A illustrates schematically a compact memory device having a bankof partitioned read/write stacks, in which the improved processor of thepresent invention is implemented.

FIG. 7B illustrates a preferred arrangement of the compact memory deviceshown in FIG. 7A.

FIG. 8 illustrates schematically a general arrangement of the basiccomponents in a read/write stack shown in FIG. 7A.

FIG. 9 illustrates one preferred arrangement of the read/write stacksamong the read/write circuits shown in FIGS. 7A and 7B.

FIG. 10 illustrates an improved embodiment of the common processor shownin FIG. 9.

FIG. 11A illustrates a preferred embodiment of the input logic of thecommon processor shown in FIG. 10.

FIG. 11B illustrates the truth table of the input logic of FIG. 11A.

FIG. 12A illustrates a preferred embodiment of the output logic of thecommon processor shown in FIG. 10.

FIG. 12B illustrates the truth table of the output logic of FIG. 12A.

FIG. 13 is a simplified version of FIG. 10 that shows some specificelements that are relevant to the present discussion in a two-bitembodiment of the present invention

FIG. 14 indicates the latch assignment for the same elements as FIG. 13for upper page program where the lower page data is read in.

FIG. 15 illustrates aspects of cache program in the single page mode.

FIG. 16 shows a programming waveform that can be used in a lower page tofull sequence conversion.

FIG. 17 illustrates the relative timing in a cache program operationwith a full sequence conversion.

FIG. 18 describes the disposition of latches in a cache page copyoperation.

FIGS. 19A and 19B illustrate the relative timings in cache page copyoperations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 7A illustrates schematically a compact memory device having a bankof partitioned read/write stacks, in which the improved processor of thepresent invention is implemented. The memory device includes atwo-dimensional array of memory cells 300, control circuitry 310, andread/write circuits 370. The memory array 300 is addressable by wordlines via a row decoder 330 and by bit lines via a column decoder 360.The read/write circuits 370 is implemented as a bank of partitionedread/write stacks 400 and allows a block (also referred to as a “page”)of memory cells to be read or programmed in parallel. In a preferredembodiment, a page is constituted from a contiguous row of memory cells.In another embodiment, where a row of memory cells are partitioned intomultiple blocks or pages, a block multiplexer 350 is provided tomultiplex the read/write circuits 370 to the individual blocks.

The control circuitry 310 cooperates with the read/write circuits 370 toperform memory operations on the memory array 300. The control circuitry310 includes a state machine 312, an on-chip address decoder 314 and apower control module 316. The state machine 312 provides chip levelcontrol of memory operations. The on-chip address decoder 314 providesan address interface between that used by the host or a memorycontroller to the hardware address used by the decoders 330 and 370. Thepower control module 316 controls the power and voltages supplied to theword lines and bit lines during memory operations.

FIG. 7B illustrates a preferred arrangement of the compact memory deviceshown in FIG. 7A. Access to the memory array 300 by the variousperipheral circuits is implemented in a symmetric fashion, on oppositesides of the array so that access lines and circuitry on each side arereduced in half. Thus, the row decoder is split into row decoders 330Aand 330B and the column decoder into column decoders 360A and 360B. Inthe embodiment where a row of memory cells are partitioned into multipleblocks, the block multiplexer 350 is split into block multiplexers 350Aand 350B. Similarly, the read/write circuits are split into read/writecircuits 370A connecting to bit lines from the bottom and read/writecircuits 370B connecting to bit lines from the top of the array 300. Inthis way, the density of the read/write modules, and therefore that ofthe partitioned read/write stacks 400, is essentially reduced by onehalf.

FIG. 8 illustrates schematically a general arrangement of the basiccomponents in a read/write stack shown in FIG. 7A. According to ageneral architecture of the invention, the read/write stack 400comprises a stack of sense amplifiers 212 for sensing k bit lines, anI/O module 440 for input or output of data via an I/O bus 231, a stackof data latches 430 for storing input or output data, a common processor500 to process and store data among the read/write stack 400, and astack bus 421 for communication among the stack components. A stack buscontroller among the read/write circuits 370 provides control and timingsignals via lines 411 for controlling the various components among theread/write stacks.

FIG. 9 illustrates one preferred arrangement of the read/write stacksamong the read/write circuits shown in FIGS. 7A and 7B. Each read/writestack 400 operates on a group of k bit lines in parallel. If a page hasp=r*k bit lines, there will be r read/write stacks, 400-1, . . . ,400-r.

The entire bank of partitioned read/write stacks 400 operating inparallel allows a block (or page) of p cells along a row to be read orprogrammed in parallel. Thus, there will be p read/write modules for theentire row of cells. As each stack is serving k memory cells, the totalnumber of read/write stacks in the bank is therefore given by r=p/k. Forexample, if r is the number of stacks in the bank, then p=r*k. Oneexample memory array may have p=512 bytes (512×8 bits), k=8, andtherefore r=512. In the preferred embodiment, the block is a run of theentire row of cells. In another embodiment, the block is a subset ofcells in the row. For example, the subset of cells could be one half ofthe entire row or one quarter of the entire row. The subset of cellscould be a run of contiguous cells or one every other cell, or one everypredetermined number of cells.

Each read/write stack, such as 400-1, essentially contains a stack ofsense amplifiers 212-1 to 212-k servicing a segment of k memory cells inparallel. A preferred sense amplifier is disclosed in United StatesPatent Publication No. 2004-0109357-A1, the entire disclosure of whichis hereby incorporated herein by reference.

The stack bus controller 410 provides control and timing signals to theread/write circuit 370 via lines 411. The stack bus controller is itselfdependent on the memory controller 310 via lines 311. Communicationamong each read/write stack 400 is effected by an interconnecting stackbus 431 and controlled by the stack bus controller 410. Control lines411 provide control and clock signals from the stack bus controller 410to the components of the read/write stacks 400-1.

In the preferred arrangement, the stack bus is partitioned into a SABus422 for communication between the common processor 500 and the stack ofsense amplifiers 212, and a DBus 423 for communication between theprocessor and the stack of data latches 430.

The stack of data latches 430 comprises of data latches 430-1 to 430-k,one for each memory cell associated with the stack The I/O module 440enables the data latches to exchange data with the external via an I/Obus 231.

The common processor also includes an output 507 for output of a statussignal indicating a status of the memory operation, such as an errorcondition. The status signal is used to drive the gate of ann-transistor 550 that is tied to a FLAG BUS 509 in a Wired-Orconfiguration. The FLAG BUS is preferably precharged by the controller310 and will be pulled down when a status signal is asserted by any ofthe read/write stacks.

FIG. 10 illustrates an improved embodiment of the common processor shownin FIG. 9. The common processor 500 comprises a processor bus, PBUS 505for communication with external circuits, an input logic 510, aprocessor latch PLatch 520 and an output logic 530.

The input logic 510 receives data from the PBUS and outputs to a BSInode as a transformed data in one of logical states “1”, “0”, or “Z”(float) depending on the control signals from the stack bus controller410 via signal lines 411. A Set/Reset latch, PLatch 520 then latchesBSI, resulting in a pair of complementary output signals as MTCH andMTCH*.

The output logic 530 receives the MTCH and MTCH* signals and outputs onthe PBUS 505 a transformed data in one of logical states “1”, “0”, or“Z” (float) depending on the control signals from the stack buscontroller 410 via signal lines 411.

At any one time the common processor 500 processes the data related to agiven memory cell. For example, FIG. 10 illustrates the case for thememory cell coupled to bit line 1. The corresponding sense amplifier212-1 comprises a node where the sense amplifier data appears. In thepreferred embodiment, the node assumes the form of a SA Latch, 214-1that stores data. Similarly, the corresponding set of data latches 430-1stores input or output data associated with the memory cell coupled tobit line 1. In the preferred embodiment, the set of data latches 430-1comprises sufficient data latches, 434-1, . . . , 434-n for storingn-bits of data.

The PBUS 505 of the common processor 500 has access to the SA latch214-1 via the SBUS 422 when a transfer gate 501 is enabled by a pair ofcomplementary signals SAP and SAN. Similarly, the PBUS 505 has access tothe set of data latches 430-1 via the DBUS 423 when a transfer gate 502is enabled by a pair of complementary signals DTP and DTN. The signalsSAP, SAN, DTP and DTN are illustrated explicitly as part of the controlsignals from the stack bus controller 410.

FIG. 11A illustrates a preferred embodiment of the input logic of thecommon processor shown in FIG. 10. The input logic 520 receives the dataon the PBUS 505 and depending on the control signals, either has theoutput BSI being the same, or inverted, or floated. The output BSI nodeis essentially affected by either the output of a transfer gate 522 or apull-up circuit comprising p-transistors 524 and 525 in series to Vdd,or a pull-down circuit comprising n-transistors 526 and 527 in series toground. The pull-up circuit has the gates to the p-transistor 524 and525 respectively controlled by the signals PBUS and ONE. The pull-downcircuit has the gates to the n-transistors 526 and 527 respectivelycontrolled by the signals ONEB<1> and PBUS.

FIG. 11B illustrates the truth table of the input logic of FIG. 11A. Thelogic is controlled by PBUS and the control signals ONE, ONEB<0>,ONEB<1> which are part of the control signals from the stack buscontroller 410. Essentially, three transfer modes, PASSTHROUGH,INVERTED, and FLOATED, are supported.

In the case of the PASSTHROUGH mode where BSI is the same as the inputdata, the signals ONE is at a logical “1”, ONEB<0> at “0” and ONEB<1> at“0”. This will disable the pull-up or pull-down but enable the transfergate 522 to pass the data on the PBUS 505 to the output 523. In the caseof the INVERTED mode where BSI is the invert of the input data, thesignals ONE is at “0”, ONEB<0> at “1” and ONE<1> at “1”. This willdisable the transfer gate 522. Also, when PBUS is at “0”, the pull-downcircuit will be disabled while the pull-up circuit is enabled, resultingin BSI being at “1”. Similarly, when PBUS is at “1”, the pull-up circuitis disabled while the pull-down circuit is enabled, resulting in BSIbeing at “0”. Finally, in the case of the FLOATED mode, the output BSIcan be floated by having the signals ONE at “1”, ONEB<0> at “1” andONEB<1> at “0”. The FLOATED mode is listed for completeness although inpractice, it is not used.

FIG. 12A illustrates a preferred embodiment of the output logic of thecommon processor shown in FIG. 10. The signal at the BSI node from theinput logic 520 is latched in the processor latch, PLatch 520. Theoutput logic 530 receives the data MTCH and MTCH* from the output ofPLatch 520 and depending on the control signals, outputs on the PBUS aseither in a PASSTHROUGH, INVERTED OR FLOATED mode. In other words, thefour branches act as drivers for the PBUS 505, actively pulling iteither to a HIGH, LOW or FLOATED state. This is accomplished by fourbranch circuits, namely two pull-up and two pull-down circuits for thePBUS 505. A first pull-up circuit comprises p-transistors 531 and 532 inseries to Vdd, and is able to pull up the PBUS when MTCH is at “0”. Asecond pull-up circuit comprises p-transistors 533 and 534 in series toground and is able to pull up the PBUS when MTCH is at “1”. Similarly, afirst pull-down circuit comprises n-transistors 535 and 536 in series toVdd, and is able to pull down the PBUS when MTCH is at “0”. A secondpull-up circuit comprises n-transistors 537 and 538 in series to groundand is able to pull up the PBUS when MTCH is at “1”.

One feature of the invention is to constitute the pull-up circuits withPMOS transistors and the pull-down circuits with NMOS transistors. Sincethe pull by the NMOS is much stronger than that of the PMOS, thepull-down will always overcome the pull-up in any contentions. In otherwords, the node or bus can always default to a pull-up or “1” state, andif desired, can always be flipped to a “0” state by a pull-down.

FIG. 12B illustrates the truth table of the output logic of FIG. 12A.The logic is controlled by MTCH, MTCH* latched from the input logic andthe control signals PDIR, PINV, NDIR, NINV, which are part of thecontrol signals from the stack bus controller 410. Four operation modes,PASSTHROUGH, INVERTED, FLOATED, and PRECHARGE are supported.

In the FLOATED mode, all four branches are disabled. This isaccomplished by having the signals PINV=1, NINV=0, PDIR=1, NDIR=0, whichare also the default values. In the PASSTHROUGH mode, when MTCH=0, itwill require PBUS=0. This is accomplished by only enabling the pull-downbranch with n-transistors 535 and 536, with all control signals at theirdefault values except for NDIR=1. When MTCH=1, it will require PBUS=1.This is accomplished by only enabling the pull-up branch withp-transistors 533 and 534, with all control signals at their defaultvalues except for PINV=0. In the INVERTED mode, when MTCH=0, it willrequire PBUS=1. This is accomplished by only enabling the pull-up branchwith p-transistors 531 and 532, with all control signals at theirdefault values except for PDIR=0. When MTCH=1, it will require PBUS=0.This is accomplished by only enabling the pull-down branch withn-transistors 537 and 538, with all control signals at their defaultvalues except for NINV=1. In the PRECHARGE mode, the control signalssettings of PDIR=0 and PINV=0 will either enable the pull-up branch withp-transistors 531 and 531 when MTCH=1 or the pull-up branch withp-transistors 533 and 534 when MTCH=0.

Common processor operations are developed more fully in U.S. patentapplication Ser. No. 11/026,536, Dec. 29, 2004, which is herebyincorporated in its entirety by this reference.

Use of Data Latches in Cache Operations

A number of aspects of the present invention make use of the datalatches of the read/write stacks described above in FIG. 10 for cacheoperations that will data in and out while the internal memory is doingother operations such as read, write, or erase. In the above-describedarchitectures, data latches are shared by a number of physical pages.For example, as on the read/write stacks of the bit lines, shared by allof the word lines, so while one operation is going on, if any of theselatches are free, they can cache data for future operations in the sameor another word line, saving transfer time as this can be hidden behindanother operation. This can improve performance by increasing the amountof pipelining of different operations or phases of operations. In oneexample, in a cache program operation, while programming one page ofdata another page of data can be loaded in, saving on transfer time. Foranother example, in one exemplary embodiment, a read operation on oneword line is inserted into a write operation on another word line,allowing the data from the read to be transferred out of the memorywhile the data write continues on.

Note that this allows data from another page in the same block, but on adifferent word line, to be toggled out (to, for example, do an ECCoperation) while the write or other operation is going on for the firstpage of data. This inter-phase pipelining of operations allows the timeneeded for the data transfer to be hidden behind the operation on thefirst page of data. More generally, this allows a portion of oneoperation to be inserted between phases of another, typically longer,operation. Another example would be to insert a sensing operationbetween phases of, say, an erase operation, such as before an erasepulse or before a soft programming phase used as the later part of theerase.

To make the relative times needed for some of the operations discussed,a set of exemplary time values for the system described above can betake as:

Data write: ˜700 μs (lower page ˜600 μs, upper page 800 μs)

Binary data write: ˜200 μs

Erase: ˜2,500 μs

Read: ˜20-40 μs

Read and toggle out data: 2 KB data, ˜80 μs; 4 KB ˜160 μs; 8 KB ˜320 μs

These values can be used for reference to give an idea of the relativetimes involved for the timing diagrams below. If have a long operationwith different phases, a primary aspect will interpose in a quickeroperation using the shared latches of the read/write stacks if latchesavailable. For example, a read can be inserted into a program or eraseoperation, or a binary program can be inserted into an erase. Theprimary exemplary embodiments will toggle data in and/or out for onepage during a program operation for another page that shares the sameread write stacks, where, for example, a read of the data to be toggledout and modified is inserted into the verify phase of the data write.

The availability of open data latches can arise in a number of ways.Generally, for a memory storing n bits per cell, n such data latcheswill be needed for each bit line; however, not all of these latches areneeded at all times. For example, in a two-bit per cell memory storingdata in an upper page/lower page format, two data latches will be neededwhile programming the lower page. More generally, for memories storingmultiple pages, all of the latches will be needed only when programmingthe highest page. This leaves the other latches available for cacheoperations. Further, even while writing the highest page, as the variousstates are removed from the verify phase of the write operation, latcheswill free up. Specifically, once only the highest state remains to beverified, only a single latch is needed for verification purposes andthe others may be used for cache operations.

The following discussion will be based on a four state memory storingtwo-bits per cell and having two latches for data on each bit line andone additional latch for quick pass write, as described in U.S. Pat. No.7,158,421. The operations of writing the lower page, or erasing, ordoing a post erase soft program are basically a binary operation andhave one of the data latches free, which can use it to cache data.Similarly, where doing an upper page or full sequence write, once allbut the highest level has verified, only a single state needs to verifyand the memory can free up a latch that can be used to cache data. Anexample of how this can be used is that when programming one page, suchas in a copy operation, a read of another page that shares the same setof data latches, such as another word line on the same set of bit lines,can be slipped in during the verify phase of the write. The address canthen be switched to the page being written, allowing the write processto pick up where it left off without having to restart. While the writecontinues, the data cached during the interpolated read can be toggledout, checked or modified and transferred back to be present for writingback in once the earlier write operation completes. This sort cacheoperation allows the toggling out and modification of the second page ofdata to be hidden behind the programming of the first page.

As a first example, a cache program operation for a two-bit memoryoperating in single page (lower page/upper page format) program mode.FIG. 13 is a simplified version of FIG. 10 that shows some specificelements that are relevant to the present discussion in a two-bitembodiment, the other elements being suppressed to simplify thediscussion. These include data latch DL0 434-0, which is connected DataI/O line 231, data latch DL1 434-1, connected to common processor 500 byline 423, data latch DL2 434-2, commonly connected with the other datalatches by line 435, and sense amp data latch DLS 214, which isconnected to common processor 500 by line 422, The various elements ofFIG. 13 are labeled according to their disposition during theprogramming of the lower page. The latch DL2 434-2 is used for the lowerverify (VL) in quick pass write mode, as is described in U.S. Pat. No.7,158,421; the inclusion of the register, and of using quick pass writewhen it is included, are optional, but the exemplary embodiment willinclude this register.

The programming of the lower page can include the following steps:

(1) The process begins by resetting data latches DL0 434-0 the defaultvalue “1”. This convention is used to simplify partial page programmingas cells in a selected row that are not to be programmed will be programinhibited.

(2) Program data is supplied to DL0 434-0 along I/O line 231.

(3) The program data will be transferred to DL1 434-1 and DL2 434-2 (ifthis latch is included and quick pass write is implemented).

(4) Once the program data is transferred to DL 1 434-1, data latch DL0434-0 can be reset to “1” and, during program time, the next data pagecan be loaded to DL0 434-0 along I/O line 231, allowing the caching of asecond page while a first page is being written.

(5) Once the first page is loaded into DL1 434-1, programming can begin.DL1 434-1 data is used for lockout of the cell from further programming.DL2 434-2 data is used for the lower verify lockout that governs thetransition to the second phase of quick pass write, as described in U.S.Pat. No. 7,158,421.

(6) Once programming begins, after a programming pulse, the result ofthe lower verify is used to update DL2 434-2; the result of the higherverify is used to update DL1 434-1. (This discussion is based on the“conventional” coding, where the lower page programming is to the Astate. This, and other codings are discussed further in U.S. Pat. Nos.7,158,421 and 7,251,160. The extension of the present discussion toother codings follows readily.)

(1) In determining of whether programming is complete, only the DL1434-1 registers of the cells of row (or appropriate physical unit ofprogram) are checked.

Note that under this arrangement, in step 6, the latch DL0 434-0 is nolonger required and can be used to cache data for the next programmingoperation. Additionally, in embodiments using quick pass write, once thesecond, slowly programming phase is entered, the latch DL2 434-2 couldalso be made available for caching data, although, in practice, it isoften the case that this is only available in this way for a fairlyshort time period that does not justify the additional overhead that isoften required to implement this feature.

FIG. 15 can be used to illustrate many of the aspects of cache programin the single page mode that has been described in the last fewparagraphs. FIG. 15 shows the relative timing of what events areoccurring internally to the memory (the lower “True Busy” line) and asseen from external to the memory (the upper “Cache Busy” line).

At time t₀ the lower page to be programmed onto the selected word line(WLn) is loaded into the memory. This assumes the first lower page ofdata has not been previously cached, as it will be for the subsequentpages. At time t₁ the lower page is finished loading and the memorybegins to write it. Since this is equivalent to a binary operation atthis point, only the state A needs to be verified (“pvfyA”) and the datalatch DL0 434-0 is available to receive the next page of data, heretaken as the upper pages to be programmed into WLn, at time t₂, which isconsequently cached in latch DL0 434-0 during the programming of thelower page. The upper page finishes loading at time t₃ and can beprogrammed as soon as the lower page finishes at t₄. Under thisarrangement, although all of the data (lower and upper page) to bewritten into physical unit of programming (here, word line WLn), thememory must wait from time t₃ to time t₄ before the upper page data canbe written, unlike the full sequence embodiment described below.

The programming of the upper page begins at time t₄, where initiallyonly the B state is verified (“pvfyB”), the C state being added at t₅(“pvfyB/C”). Once the B state is no longer being verified at t₆, onlythe C state needs to be verified (“pvfyC”) and the latch DL0 434-0 isfreed up. This allows the next data set to be cached while the upperpage finishes programming.

As noted, according to the single page algorithm with cache program, asshown in FIG. 15, even though the upper page data may be available attime t₃, the memory will wait until time t₄ before starting to writethis data. In a conversion to a full sequence program operation, such asis developed more fully in U.S. patent application Ser. No. 11/013,125,once the upper page is available the upper and lower page data can beprogrammed concurrently.

The algorithm for cache program in full sequence (low to fullconversion) write begins with lower page program as above. Consequently,steps (1)-(4) are as for the lower page process in single page programmode:

(1) The process begins by resetting data latches DL0 434-0 the defaultvalue “1”. This convention is used to simplify partial page programmingas cells in a selected row that are not to be programmed will be programinhibited.

(2) Program data is supplied to DL0 434-0 along I/O line 231.

(3) The program data will be transferred to DL1 434-1 and DL2 434-2 (ifthis latch is included and quick pass write is implemented).

(4) Once the program data is transferred to DL 1 434-1, data latch DL0434-0 can be reset to “1” and, during program time, the next data pagecan be loaded to DL0 434-0 along I/O line 231, allowing the caching of asecond page while a first page is being written.

Once the second page of data is loaded, if correspond to the upper ofthe lower page being written and the lower page is not yet finishedprogramming, the conversion to full sequence write can be implemented.This discussion focuses on the use of the data latches in such analgorithm, with many of the other details being developed more full inco-pending, commonly assigned U.S. patent application Ser. No.11/013,125.

(5) After the upper page data is loaded into latch DL0 434-0, a judgmentwill be done in the address block to check if the 2 pages are on thesame word line and the same block, with one page is the lower page andone is upper page. If so, then the program state machine will trigger alower page program to full sequence program conversion if this isallowed. After any pending verify is complete, the transition is theneffected.

(6) Some operation parameters will be typically be changed when theprogram sequence changed from lower page to full sequence. In theexemplary embodiment these include:

-   -   (i) Maximum program loop for the number of pulse verify cycles        will be changed from that of the lower page algorithm to that of        the full sequence if the lower page data has not been locked        out, but the number of program loops completed will not be reset        by the conversion.    -   (ii) As shown in FIG. 16, the programming waveform starts with        the value VPGM_L used in the lower page programming process. If        the programming waveform has progressed to where it exceeds the        beginning value VPGM_U used in the upper page process, at        conversion to full sequence, the staircase will drop back down        to VPGM_U prior to continuing up the staircase.    -   (iii) The parameters determining the step size and maximum value        of the program pulse are not changed.

(7) A full sequence read of the current state of the memory cells shouldbe performed to guarantee the right data will be programmed formulti-level coding. This ensures that states that may have formerlylocked out in the lower page programming, but which require furtherprogramming to take account of their upper page data, are not programinhibited when the full sequence begins.

(8) If quick pass write is activated, the data of latch DL2 434-2 willbe updated as well to reflect the upper page program data, since thiswas formerly based on the lower verify for only the A state.

(9) The programming then resumes with the multi-level, full sequenceprogram algorithm. If the program waveform in the lower page process hasincreased beyond the upper page starting level, the waveform is steppedback to this level at conversion time, as shown in FIG. 16.

FIG. 17 is a schematic representation of the relative times involved inthe lower page to full sequence conversion write process. Up until timet₃, the process is as described above for the process in FIG. 15. At t₃the upper page of data has been loaded and the transition is made to thefull sequence algorithm the verification process is switched to includethe B states with the A states. Once all of the A states lock out, theverify process switches to checking for the B and C states at time t₄.Once the B states have verified at t₅, only the C state needs to bechecked and a register can be freed up to load the next data to beprogrammed, such as the lower page on the next word line (WL_(n+1)) asindicated on the Cache Busy line. At time t₆ this next data set has beencached and one the programming of the C data for the previous setconcludes at t₇, this next data set begins programming. Additionally,while the (here) lower page on word line WL_(n+1) is programming, thenext data (such as the corresponding upper page data) can be loaded intothe open latch DL0 434-0.

During the full sequence write, a status report is implemented in a waythat gives lower page and upper page status independently. At the end ofthe program sequence, if there are unfinished bits, a scan of physicalpage can be performed. A first scan can check latch DL0 434-0 forunfinished upper page data, a second scan can check DL1 434-1 forunfinished lower page data. Since, the verification of the B state willchange both DL0 434-0 and DL1 434-1 data, an A state verification shouldbe performed in the way that DL1 434-1 data “0” will be changed to “1”if the bit's threshold value is higher than the A verify level. Thispost verify will check on whether any under programmed B levels arepassing at the A level; if they are passing at the A level, then theerror is only on upper page and not on lower page; if they are notpassing at the A level, then both lower and upper pages have error.

If the cache program algorithm is used, after the A and B data areprogrammed, the C state will be transferred to latch DL1 434-1 to finishprogramming. In this case, the scan of latch is not necessary for lowerpage, because the lower page will have already passed program withoutany failed bits.

Another set of exemplary embodiments of the present invention relate topage copy operations, where a data set is relocated from one location toanother. Various aspects of data relocation operations are described inU.S. patent applications number U.S. Ser. No. 10/846,289, filed May 13,2004, Ser. No. 11/022,462, Dec. 21, 2004; and number U.S. Ser. No.10/915,039, filed Aug. 9, 2004; and U.S. Pat. No. 6,266,273, which areall hereby incorporated by reference, which are all hereby incorporatedby reference. When data is copied from one location to another, the datais often toggled out to be checked (for error, for example), updated(such as updating a header), or both (such correcting detected error).Such transfers are also to consolidate date in garbage collectionoperations. A principal aspect of the present invention allows for adata read to an open register to be interpolated during the verify phaseof a write operation, with this cached data then being transferred outof the memory device as the write operation continues, allowing the timefor toggling the data out to hide behind the write operation.

The following presents two exemplary embodiments of a cache page copyoperation. In both cases, an implementation that uses a quick pass writeimplementation is described. FIG. 18 indicates the disposition of theexemplary arrangement of latches as the process progresses.

The first version of cache page copy will write to a lower page and caninclude the following steps, where read addresses are labeled M, M+1, .. . , and write addresses are labeled N, N+1, . . . :

(1) The page to be copied (“page M”) is read into latch DL1 434-1. Thiscan be either an upper or lower page of data

(2) Page M is then transferred into DL0 434-0.

(3) The data in DL0 434-0 is then toggle out and modified, after whichit is transferred back into the latch.

(4) The program sequence can then begin. After data to be written intothe lower page N is transferred to DL1 434-1 and DL2 434-2, the latchDL0 434-0 is ready for cache data. This lower page will be programmed.For this embodiment, the program state machine will stop here.

(5) The next page to be copied is then read into DL0 434-0. Programmingcan then resume. The state machine, stopped at the end of step (4), willrestart the program sequence from the beginning.

(6) Programming continues until the lower page finishes.

The copy destination page address will determine whether a write is to alower or an upper page. If the program address is an upper page address,then the programming sequence will not be stopped until the programmingfinishes and the read of step (5) will be executed after the write iscomplete.

In a second cache page copy method, the program/verify process can bepaused to insert a read operation and then restart the write operation,picking up at the point where it left off. The data that was read duringthis interleaved sensing operation can then be toggled out while theresumed write operation continues on. Also, this second process allowsfor the page copy mechanism to be used in an upper page or full sequencewrite process once only the C state is being verified and one latch oneach bit line opens up. The second cache page copy operation begins withthe same first three steps as in the first case, but then differs. Itcan include the following steps:

(1) The page to be copied (“page M”) is read into latch DL1 434-1. Thiscan be either a lower or upper page

(2) The data from page M is then transferred into DL0 434-0. (As before,N, etc. will denote a write address, M, etc., for a read address.)

(3) The data in DL0 434-0 is then toggled out, modified, and thentransferred back to the latch.

(4) The state machine program will go to an infinite wait state untilthe command a read command is entered and then a read of another page,say the next page M+1, to latch DL0 434-0 will begin.

(5) Once the read of step (4) is complete, the address is switched backto word line and block address to program the data in steps (1-3) intopage N (here, a lower page) and the programming is resumed.

(6) After the read of page M+1 is finished, the data can be toggled out,modified, and returned. Once the process is complete, the write can beconverted to a full sequence operation if the two pages are thecorresponding upper and lower pages on the same WL.

(7) Once the A and B levels are done in the full sequence write, thedata in DL0 434-0 will be transferred to DL1 434-1, as in the normalcache program described earlier, and a read command for another page(e.g., page M+2) can be issued. If there is not a single page to fullsequence conversion, the lower page will finish the writing and then theupper page will start. After the B level state is done completely, thesame DL0 434-0 to DL1 434-1 data transfer will occur, and the statemachine will go into state of waiting for the read command for page M+2.

(8) Once the read command arrives, the address is switched to the readaddress and the next page (page M+2) is read out.

(9) Once the read is complete, the address will be switched back toprevious upper page address (program address N+1) until the writefinishes.

As noted above, the exemplary embodiments include the latch DL2 434-2used for the lower verify of the quick pass write technique in additionto the latches DL0 434-0 and DL1 434-1 used in holding the (here, 2bits) of data that can be programmed into each of the memory cells. Oncethe lower verify is passed, the latch DL2 434-2 may also be freed up andused to cache data, although this is not done in the exemplaryembodiments.

FIGS. 19A and 19B illustrate the relative timing of the second cachepage copy method, where FIG. 19B illustrates the algorithm with the fullsequence write conversion and FIG. 19A illustrates the algorithmwithout. (Both FIGS. 19A and 19B are composed of two parts, the first,upper part beginning at the broken vertical line A, corresponding t₀,and ending with the broken vertical line B, corresponding to t₅; thesecond, lower part is a continuation of the upper portion and beginswith the broken vertical line B, corresponding to t₅. In both cases theline B at time t₅ is same in the upper portion as in the lower portion,being just a seam in two parts allowing it to be displayed on twolines.)

FIG. 19A shows a process that starts with reading of a first page (pageM) that is taken to be a lower page in this example, assumes no data haspreviously been cached, and operates in single page mode, waiting untilthe lower page has finished writing before beginning to write the upperpage. The process starts at time t₀ with a read of the page M (Sensepage M (L)), which here is a lower that is sensed by a read at the A andC levels in this coding. At time at time t₁ the read is complete andpage M can be toggled out and checked or modified. Beginning at time t₂a next page (here page M+1, the upper page corresponding to the samephysical as lower page M) is sensed by reading at the B level, a processthat finishes at time t₃. At this point, the first page (originatingfrom Page M) (lower) is ready to be programmed back into the memory atpage N and the data read from page M+1 is being held in a latch and canbe transferred out to be modified/checked. Both of these processes canstart at the same time, here t₃. Using the typical time values describedabove, the data from page M+1 has been toggled out and modified by timet₄; however, for the embodiment not implementing a full sequenceconversion, the memory will wait until page N finishes at time t₅ tobegin writing the second read page of data (originating from Page M+1)into page N+1.

As page N+1 is an upper page, its write begins initially with averification at the B level, the C level being added at t₆. Once thestorage elements having a target state B all lock out (or the maximumcount is reached) at time t₇, the B state verification is dropped. Asdescribed above, according to several principal aspects of the presentinvention, this allows a data latch to be freed up, an ongoing writeoperation is suspended, a reading operation (at a different address thanthe suspended program/verify operation) is interposed, the write thenresumes where it left off, and the data sensed the interposed writeoperation can be toggled out while the resumed write operation runs on.

At time t₇ the interposed write operation is performed for the, here,lower page M+2. This sensing is finished at time t₈ and the write ofpage N+1 picks back up and the data from page M+2 is concurrentlytoggled out and modified. In this example, page N+1 finishes programmingat time t₉ before page M+2 is finished at time t₁₀. At time t₁₀ a writeof the data originating from page M+2 could begin; however, in thisembodiment, instead a read of page M+3 is first executed, allowing forthis page's data to be toggled out and the modification to be hiddenbehind the writing of the data originating from page M+2 into page N+2,beginning at time t₁₁. The process then continues on as in the earlierparts of the diagram, but with the page numbers shifted, with time t₁₁corresponding to time t₃, time t₁₂ corresponding to time t₄, and so onuntil the copy process is stopped.

FIG. 19B again shows a process that starts with reading of a lower page,page M that is taken to be a lower page, and assumes no data haspreviously been cached. FIG. 19B differs from FIG. 19A by implementing aconversion to full sequence write at time t₄. This roughly speeds up theprocess by the time (t₅−t₄) of FIG. 19A. At time t₄ (=t₅ in FIG. 19A),the various changes related to the full sequence conversion areimplemented as described previously. Otherwise, the process is similarto that of FIG. 19A, including those aspects of the present inventionfound between times t₇ and t₁₂.

In both the page copy processes and the other techniques described herethat involve writing data, which states are verified at a given time canbe selected intelligently, along the lines describe in U.S. patentapplication Ser. No. 10/314,055, filed Dec. 5, 2002, which is herebyincorporated by reference. For example, in the full sequence write, thewrite process can begin verifying only the A level. After ever A verify,it is checked to see whether any bits have passed. If so, the B levelcan be added to the verify phase. The A level verify will be removedafter all storage units with it as their target values verify (or excepta maximum count based on a settable parameter). Similarly, after theverifications at the B level, a verify of the C level can be added, withthe B level verify being removed after all storage units with it astheir target values verify (or except a maximum count based on asettable parameter).

Although the various aspects of the present invention have beendescribed with respect to certain embodiments, it is understood that theinvention is entitled to protection within the full scope of theappended claims.

1. A method of operating a non-volatile memory device which includes anarray of memory cells and a set of read/write circuits for operating ona group of memory cells of said array in parallel, each read/writecircuits having a set of data latches for latching input and/or outputdata of a corresponding one of said group of memory cells, the methodcomprising: loading a first data set into a first plurality of the datalatches; performing a first operation on a first group of memory cellsusing all of the latches of the first plurality of the sets of datalatches; during said first operation, releasing some, but less than all,of the latches of the first plurality of the sets of data latches; andduring said first operation, caching a second data set for a secondoperation in the released ones of said first plurality of the sets ofdata latches.
 2. The method of claim 1, wherein said second data set issupplied from external to the memory device.
 3. The method of claim 1,wherein the first operation includes multiple phases and said seconddata set is read from a second group of memory cells distinct from thefirst group of memory cells, the second data set being read betweenphases of the first operation.
 4. The method of claim 3, wherein thefirst operation is a write operation having alternating program andverify phases and the first set of data is the data to be written intothe first group of memory cells.
 5. The method of claim 4, wherein thecached data set is subsequently transferred out of said first pluralityof the sets of data latches during the write operation.
 6. The method ofclaim 4, wherein the alternating program and verify phases resume wherewrite process was paused while the second data set was read.
 7. Themethod of claim 4, wherein said memory cells are multi-level memorycells storing N bits of data, where N is greater than one, and whereineach of said sets of data latches includes N data latches and said firstdata set is N bit data.
 8. The method of claim 3, wherein the first andsecond data sets correspond to distinct first and second word lines ofthe memory.
 9. The method of claim 1, wherein each of said sets oflatches is associated with a distinct bit line of the memory.
 10. Amethod of operating a non-volatile memory device which includes an arrayof memory cells and a set of read/write circuits for operating on agroup of memory cells of said array in parallel, each read/writecircuits having a set of data latches for latching input and/or outputdata of a corresponding one of said group of memory cells, the methodcomprising: storing a first set of data for a first group of the memorycells into the corresponding sets of data latches; writing the first setof data into said first group of memory cells using all of thecorresponding sets of data latches, wherein the writing includesalternating program and verify phases; during the writing of the firstset of data, releasing some, but less than all, of the correspondingsets of data latches pausing said writing between one of the programphases and one of the verify phases; subsequent to said pausing, readinga second set of data from a second group of the memory cells into thereleased latches of said corresponding sets of data latches; andsubsequent to said reading, resuming the paused writing.
 11. The methodof claim 10, wherein the writing is managed by a state machine, saidpausing is in response to receiving a command for said reading, and theresuming is in response to completing the reading.
 12. The method ofclaim 10, further comprising: subsequent to said reading, transferredthe second set of data out of the memory during the resumed writing.